Due to the recent toughening of exhaust gas controls and demands for reducing noise, there has been a rising demand for optimizing combustion in the combustion chambers of diesel engines as well. To optimize combustion, accurate control of the fuel injection amount, fuel injection timing, injection period, etc. becomes necessary even in diesel engines.
However, in a diesel engine, generally the amount of intake air is not adjusted. The engine load is controlled by the fuel injection amount. Therefore, in a diesel engine, combustion is performed in a lean air-fuel ratio region considerably higher than the stoichiometric air-fuel ratio. Further, the air-fuel ratio changes depending on the load. Therefore, in a conventional diesel engine, the air-fuel ratio is not strictly controlled as in a gasoline engine. In the past, the fuel injection amount, fuel injection timing, and other fuel injection parameters also have not been controlled as strictly as with a gasoline engine. Further, in the past, in a diesel engine, the target values of the fuel injection amount, injection timing, injection pressure, and other fuel injection parameters have been determined from the engine operating conditions (speed, accelerator opening degree, etc.) and the fuel injector has been open-loop controlled in accordance with those target values. With open loop control, however, it was impossible to prevent error in the actual fuel injection amount compared with the target fuel injection amount and was difficult to accurately control the combustion state to the targeted state.
Further, to improve the exhaust gas properties and reduce the noise, multi-fuel injection injecting fuel a plurality of times before and after the main fuel injection so as to optimally adjust the combustion state is effective. However, for multi-fuel injection, it is necessary to precisely control the fuel injection amounts and injection timings of the plurality of fuel injections.
Further, in a common rail type high pressure fuel injection system designed to be employed in recent diesel engines for improving the combustion state, since the fuel injection period is short and the fuel injection pressure changes during the injection, there is the problem of a susceptibility to error in the fuel injection amount. Therefore, in a common rail type high pressure fuel injection system, measures have been adopted such as setting the tolerance of the fuel injectors small so as to improve the fuel injection accuracy, but in practice fuel injectors change in fuel injection characteristics along with the period of use due to wear of the parts etc., so with open loop control, it is difficult to make the fuel injection parameters constantly accurately match with the target values.
In this way, in a diesel engine, error is liable to occur in the fuel injection amount etc., so even if setting target values giving the optimal combustion state, in practice sometimes making the fuel injection amount match with the target value is difficult.
On the other hand, it is known in the art to reduce the combustion temperature of an engine so as to reduce the NOx and other harmful emissions by recirculating part of the engine exhaust gas to the engine combustion chambers as EGR gas. Further, even in a diesel engine engaged in normal lean air-fuel ratio operation, it is possible to supply a relatively large amounts of EGR gas to the combustion chamber so as to reduce the harmful emissions in the exhaust gas.
However, EGR gas has a large effect on combustion. In particular, in a diesel engine, the amount of EGR gas has a large effect on the ignition delay time from the start of fuel injection to when the injected fuel starts to burn. Therefore, if EGR gas is excessively supplied to the combustion chamber, the engine combustion state will deteriorate and a drop in the engine performance and deterioration of the exhaust gas properties will occur.
On the other hand, if the amount of EGR gas is small, the effect of suppression of the harmful emissions will fall. Therefore, the amount of EGR gas has to be controlled to a suitable amount in accordance with the operating conditions of the engine.
However, conventionally the amount of EGR gas has not been controlled precisely. In particular, in a diesel engine, the opening degree of an EGR valve for controlling the flow rate of EGR gas usually was open loop controlled to a value determined from the engine speed and accelerator opening degree (amount of depression of accelerator pedal).
However, due to the recent toughening of exhaust gas controls and demands for reducing noise, a need has arisen to precisely control even the EGR gas flow rate to its optimal value. If precisely controlling the EGR in this way, a sufficient accuracy cannot be obtained with open loop control based on the engine speed and accelerator opening degree like in the past. Further, for example, it is possible to arrange an air-fuel ratio sensor in the engine exhaust passage and to control the amount of EGR gas based on the exhaust air-fuel ratio detected by the air-fuel ratio sensor, but with an engine like a diesel engine which is operated in a state where the exhaust air-fuel ratio is extremely lean, the detection accuracy of the air-fuel ratio sensor falls, so there is the problem that if controlling the amount of EGR gas based on the exhaust air-fuel ratio detected by the air-fuel ratio sensor, the error becomes large.
Even in control of the amount of EGR gas, it is effective to detect the actual combustion state by some form or another and feedback control the amount of EGR gas so that the actual combustion state matches with the targeted combustion state.
That is, while conventionally the fuel injection amount, fuel injection timing, amount of EGR gas, etc. have been open loop controlled based on the engine speed and accelerator opening degree, to improve the properties of the exhaust gas and reduce the noise, it becomes necessary to accurately feedback control the fuel injection amount, fuel injection timing, amount of EGR gas, etc. based on the actual combustion state.
Control of the fuel injection or EGR based on the actual engine combustion state is for example described in Japanese Unexamined Patent Publication (Kokai) No. 2000-54889.
The system of Japanese Unexamined Patent Publication (Kokai) No. 2000-54889 does not relate to a diesel engine, but relates to a gasoline engine, but uses the heat release (heat generation) rate in the combustion chamber as a combustion parameter expressing the combustion state of the engine and controls the flow rate of EGR gas and the fuel injection timing, fuel injection amount, ignition timing, etc. so that the heat release rate becomes a predetermined pattern.
That is, the system of Japanese Unexamined Patent Publication (Kokai) No. 2000-54889 arranges a cylinder pressure sensor for detecting the pressure inside an engine combustion chamber in each cylinder, calculates the heat release rate at each crank angle based on the detected actual pressure inside the combustion chamber (combustion pressure) and crank angle, and feedback controls the amount of EGR gas, ignition timing, fuel injection timing, etc. so that the pattern of change of the heat release rate with respect to the crank angle matches a predetermined ideal pattern of change in accordance with the operating conditions and thereby obtain the optimal combustion.
The system of Japanese Unexamined Patent Publication (Kokai) No. 2000-54889 takes note of the heat release rate as a parameter relating to combustion, calculates the pattern of the heat release rate in actual operating conditions, and makes the heat release rate follow a predetermined pattern by feedback controlling the ignition timing, fuel injection amount, etc. The system of Japanese Unexamined Patent Publication (Kokai) No. 2000-54889 relates to a gasoline engine, but it may be considered to similarly provide cylinder pressure sensors in a diesel engine as well and thereby calculate the pattern of the heat release rate based on the outputs of the cylinder pressure sensors and feedback control the fuel injection timing and fuel injection amount so that the peak positions or pattern of the heat release rate becomes predetermined peak positions or pattern of heat release rate.
However, the system of Japanese Unexamined Patent Publication (Kokai) No. 2000-54889 uses only the heat release rate in a combustion chamber as a parameter expressing the combustion state of the engine for feedback control of the combustion state. The system of Japanese Unexamined Patent Publication (Kokai) No. 2000-54889 is used by a gasoline engine. In a gasoline engine, the pre-mixed air-fuel mixture is ignited by sparks. The ignition, combustion, and other combustion parameters also do not change much. Therefore, no great error occurs even if using only the peak positions or pattern of the heat release rate as a parameter expressing the combustion state.
In a diesel engine however, for example, sometimes not only main fuel injection, but also multi-fuel injection including pilot injection performed before the main fuel injection, after injection performed after the main fuel injection, etc. is performed. Even looking at just the type of injection (injection mode), sometimes there is a large difference. Further, in a diesel engine, the combustion pattern (combustion mode) changes depending on the amount of EGR gas.
For this reason, since the change in pressure in a combustion chamber greatly differs depending on the injection mode or combustion mode as well, feedback control of the combustion state by just the peak positions or pattern of the heat release rate is not necessarily suitable.
For example, in an in-cylinder fuel injector of a diesel engine, the injection amount, injection timing, and other fuel injection characteristics gradually change along with the period of use resulting in deviation in fuel injection characteristics, but such deviation in fuel injection characteristics is difficult to accurately correct based on the peak positions or pattern of the heat release rate.
Further, when performing pilot injection or main fuel injection or after injection or other multi-fuel injection, optimization of the combustion state requires optimal control of the fuel injection amount and injection timing of the fuel of each, but feedback control of the fuel injection characteristics of a plurality of fuel injections is difficult based on only the peak positions or pattern of the heat release rate.
On the other hand, as an example of a combustion control system of an internal combustion engine using a parameter other than the heat release rate to detect the combustion state and controlling the fuel injection characteristics of multi-fuel injection in accordance with the combustion state, there is the one described in Japanese Unexamined Patent Publication (Kokai) No. 2001-123871.
The system of Japanese Unexamined Patent Publication (Kokai) No. 2001-123871 measures the combustion noise of a diesel engine, judges whether the pilot injection amount is too great based on the measured combustion noise, and corrects the pilot injection amount based on this. Further, as the combustion noise, it uses the derivative or second derivative of the cylinder pressure detected by a cylinder pressure sensor detecting the pressure inside a combustion chamber so as to remove the effect of mechanical vibration and thereby improve the detection accuracy of the combustion noise.
That is, the system of Japanese Unexamined Patent Publication (Kokai) No. 2001-123871 feedback controls the pilot injection amount based on the actually measured combustion noise so as to keep the combustion noise below a target level at all times.
As explained above, since the system of Japanese unexamined Patent Publication (Kokai) No. 2001-123871 feedback controls the pilot injection amount based on the actually measured combustion noise, it can keep the combustion noise below a target level at all times. However, while the system of Japanese Unexamined Patent Publication (Kokai) No. 2001-123871 keeps the combustion noise below a target value, it does not necessarily always obtain a good combustion state. Conversely, sometimes it deteriorates the exhaust properties.
That is, to obtain good exhaust properties, it is necessary to suitably control not only the injection amount of the pilot injection, but also the injection timing, but the system of Japanese Unexamined Patent Publication (Kokai) No. 2001-123871 controls only the injection amount of the pilot injection based on the combustion noise and does not control the injection timing based on the actual combustion state. Therefore, the system of Japanese Unexamined Patent Publication (Kokai) No. 2001-123871 has the problem that while the combustion noise falls, the exhaust properties are not always improved.
Further, the system of Japanese Unexamined Patent Publication (Kokai) No. 2001-123871 deals only with pilot injection and even more so only operation with just one pilot injection, so has the problem that it cannot suitably control the injection amounts and injection timings of the different fuel injections in multi-fuel injection consisting of a plurality of pilot injections or after injection performed after main fuel injection.